This invention relates in general to anti-lock brake systems and in particular to a driver circuit having two series connected Field Effect Transistors (FET's).
An Anti-lock Brake System (ABS) is often included as standard or optional equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes. A typical ABS includes a plurality of solenoid valves mounted within a control valve and connected to the vehicle hydraulic brake system. Usually, a separate hydraulic source, such as a motor driven pump, is included in the ABS for reapplying hydraulic pressure to the controlled wheel brakes during an ABS braking cycle. An ABS further includes an electronic control module which is electrically connected to wheel speed sensors mounted adjacent to the controlled wheels, the solenoid valve coils and the pump motor. The control module can be mounted directly upon the control valve or located remotely therefrom. The control module includes a microprocessor which is programmed to control the ABS in accordance with a control algorithm and permanently stored parameters.
During vehicle operation, the microprocessor in the ABS control module continuously receives speed signals from the wheel speed sensors. The microprocessor monitors the speed signals for potential wheel lock-up conditions. When the vehicle brakes are applied and the microprocessor senses an impending wheel lock-up condition, the microprocessor is responsive thereto to close a fail-safe power relay. The power relay, which can be an electromechanical or solid state device, controls the supply of electric power to enable the solenoid valves and, optionally, the pump motor. Additionally, if the microprocessor detects a malfunction in the ABS, the power relay is opened to deactivate the ABS. The microprocessor then selectively actuates the solenoid valves in the control valve in accordance with the control algorithm to cyclically relieve and reapply hydraulic pressure to the controlled wheel brakes. The hydraulic pressure applied to the controlled wheel brakes is adjusted by the operation of the solenoid valves to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle.
Referring now to the figures, there is illustrated in FIG. 1, a schematic drawing of a portion of a typical ABS control module which includes an electromechanical relay 10. The relay includes a relay coil 11 and a set of normally open relay contacts 12. When the relay coil 11 is energized, the relay contacts 12 close. The relay contacts 12 have a first terminal 15 connected to a vehicle power supply, which is shown as a battery in FIG. 1. A second terminal 16 of the relay contacts 12 is connected to a plurality of loads 20, two of which are shown. The loads 20 are typically the coils for the ABS solenoid valves but also can optionally include the pump motor. Each load is connected through a corresponding load driver 21 to a vehicle ground 22. The load drivers 21 will be further described below.
One end of the relay coil 11 is connected through a diode 13 to the vehicle power supply 14. The other end of the relay coil 11 is connected through a relay driver 25 to a relay output port 26 of an ABS microprocessor 27. The relay driver 25 is responsive to the voltage appearing at the relay output port 26 to energize the relay coil 11. As described above, energization of the relay coil 11 closes the relay contacts 12 to supply power to the loads 20.
Each of the load drivers 21 typically includes a power rated Field Effect Transistor (FET) which has a drain terminal (not shown) connected to the corresponding load 20 and a source terminal (not shown) connected to a vehicle ground 22. If the load driver 21 includes a logic level FET, a FET gate terminal (not shown) is connected directly to a corresponding load driver output port 28 on the microprocessor 27. Alternately, a conventional FET driver circuit (not shown) can be included in the load driver 21. The FET driver circuit is connected between the corresponding driver output port 28 and the load driver FET gate terminal. The FET driver circuit is responsive to the logic level voltages which appear at the driver output port 28 to generate a gate voltage which is sufficient to cause the load driver FET to change from its non-conducting state to its conducting state.
A schematic diagram of a typical prior art solid state relay 30 is shown in FIG. 2. Components in FIG. 2 which are similar to components shown in FIG. 1 have the same numerical designations. The solid state relay 30 includes a FET 31 having a source terminal 32 connected to a plurality of loads 20. The solid state relay 30 also includes a diode 35 having a cathode connected to a drain terminal 36 of the FET 31. The diode 35 has an anode connected to the vehicle power supply 14. The diode 35 protects the FET 31 from application of a reverse power supply voltage, which can occur if the vehicle battery is incorrectly connected following vehicle servicing. It is also known to substitute a Shott kLy diode (not shown) for the diode 35 to provide reverse voltage protection.
The FET 31 has a gate terminal 38 connected to a FET gate driver 40. The gate driver 40 is connected to the relay output port 26 of the ABS microprocessor 27. A logic signal, which alternates between ground potential and a "high" voltage, appears at the relay output port 26. Typically, the high voltage is five volts. The FET gate driver 40 is responsive to the logic signal to generate a corresponding gate signal at the FET gate terminal 38. The FET gate driver 40 applies a voltage to the FET gate terminal 38 when the microprocessor relay output port 26 is high. The gate voltage has a sufficient magnitude to cause the FET 31 to be in its conducting state. Because the FET 31 is connected to the high side of the loads 20, the gate driver 40 typically is a commercially available integrated circuit which includes a voltage doubler or charge pump for generating the voltage to be applied to the FET gate terminal 38. Alternately, the FET gate driver 40 will pull the FET gate terminal 38 to ground when the relay output port 26 is at ground. When the gate terminal 38 is at ground, the FET 31 will be in its non-conducting state.
The voltage output level of the vehicle power supply 14 can vary during operation of the vehicle. Accordingly, the FET gate driver 40 includes an overvoltage shut down circuit (not shown). The overvoltage shutdown circuit monitors the voltage at a voltage sensing port 41 which is electrically connected by a line 42 to the high side of the power supply 14. If the supply voltage exceeds a predetermined amount, the overvoltage circuit causes the FET gate driver 40 to shut down, pulling the gate terminal 38 of the FET 31 to ground. This switches the FET 31 to its non-conducting state and blocks current flow through the FET 31, protecting the FET 31 from damage. Additionally, a surge suppresser 45 is connected between the power supply 14 and the vehicle ground 22. The surge suppresser 45 is operative to prevent voltage spikes from damaging the ABS electronic components.
The operation of the driver circuit 30 will now be explained. When the microprocessor relay output port 26 is at ground, the gate driver 40 pulls the gate terminal 38 to ground. When the gate terminal 38 is at ground potential, the FET 31 is in a non-conducting state and blocks any current flow from the power supply 14 to the loads 20. When the microprocessor relay output port 26 goes high, a corresponding voltage is generated by the FET gate driver 40 and applied to the FET gate terminal 38. As explained above, the generated gate voltage is sufficiently high to cause the FET 31 to switch to its conducting state, allowing current to flow from the vehicle power supply 14 to the loads 20.